GEO 101

Composite volcanoes are found only in which tectonic setting?

subduction zone

Viscosity of magma is controlled by

temperature and silica content of the magma

When two oceanic plates collide:

The older plate will subduct

Some minerals have crystal structures with distinct weaknesses. These minerals tend to break in certain directions along planes of weakness in their structure. These types of minerals are said to possess good ______.

Cleavage

Explain why geologists believe that the Earth's core is made mostly of iron.

The average density of the Earth, as shown by its gravitational field, is significantly higher than the density of the mantle, which is known from a few direct observations of materials erupted from volcanoes and exposures of mantle rock at the surface. This is despite the fact that the mantle makes up most of the earth by volume. Therefore, the core must be very dense to explain the earth's average density. Iron is one element that would provide the needed density, and iron is known to be abundant in the solar system from studies of meteorites, making iron a likely candidate for the composition of the core.

The locations and ages of islands in the Hawaiian Island chain can be used to show

the rate of movement of the Pacific Plate. (The Hawaiian Islands have been built by the passage of the Pacific plate over a mantle hotspot. The location and ages of the islands show the direction of movement of the Pacific plate as it passed over the hotspot. This is called a hot spot track. Because the Hawaiian islands are on the Pacific Plate they tell us nothing about the direction or rate of movement of the North American plate - the North American plate moves independently of the Pacific plate. They also don't tell us anything about the size of earthquakes to be expected in the Pacific Ocean - while passage of the plate over the hotspot, and subsidence as it moves off the hot spot results in earthquake activity, the largest earthquakes occur on the subduction zones that ring the Pacific basin.)

Which property can be used to distinguish hematite from magnetite?

Streak

The magma composition erupted by a hotspot beneath oceanic crust is

basalt

Which of the following are a result of planetary differentiation?

All of the above: Light elements rose to form the crust; Heavy elements sank to form the core; Light lithosphere floating on denser asthenosphere

Lava with low viscosity is found at which volcano?

Mauna Loa, Hawaii

A hotspot is

an area of volcanic activity with a deep magma source

Which choice best describes the age distribution of oceanic crust?

oceanic crust is youngest at the ridge and gets older with increasing distance from the ridge

Why are silicate minerals the most common group of minerals in the crust?

because oxygen and silicon are the most common elements in the crust

Which of the following plate boundaries is an example of an advanced stage of continental rifting that is developing into a new ocean basin?

African Plate and Arabian Plate

How are biotite and muscovite different?

Biotite contains Fe and Mg in addition to K, while muscovite has only K

Because earth is a closed system:

Both A and B: there is a finite supply of resources and all waste and pollutants generated on Earth stay on Earth

In what ways does continental crust differ from oceanic crust?

Continental crust differs from oceanic crust in all of the following ways: its thicker, less dense, more buoyant, richer in light elements like Na, K, and Si, and poorer in dense elements like Fe and Mg relative to oceanic crust

Earth's interior is hot because

Earth's interior is hot because the process that formed Earth (accretion) generated large amounts of heat. Additional heat has been added through geologic time by radioactive decay of unstable isotopes (heat is a byproduct of the decay process). Molten rock or magma is actually rare in earth's interior - only the outer core is hot enough to be above the melting point for material of that composition under that amount of pressure.

The layer of the Earth that is thought to be a liquid is the:

Outer core

Buoyant materials are added to the edge of continents by:

Subduction of the surrounding oceanic crust

During fractional crystallization

all of the above:iron, magnesium, and calcium is removed from the magma and the magma becomes enriched in silica, sodium, and potassium and the composition of the magma becomes more felsic

Why do composite volcanoes consist of alternating lava and pyroclastic layers?

composite cones are created by a mixture of explosive activity and lava flows

Magma that is high in silica, Na, and K; and low in Fe, Mg, and Ca, is said to have what composition?

felsic

Tectonic plates of rigid and brittle __________ move over a softer and ductile ___________ below.

lithosphere; asthenosphere

A poryphyritic texture indicates an igneous rock that

partially cooled underground and was then erupted on the surface

Which group of igneous rocks is all extrusive?

pumice, basalt, andesite

Igneous rocks produced by explosive volcanic activity will have what kind of texture?

pyroclastic

Subduction zones were discovered when

when it was noticed that deep earthquakes are found only near ocean trenches, and that earthqauakes increase in depth with distance from the trench (Remember from Lecture 3 that earthquakes are not randomly distributed across the globe. Earthquakes are concentrated along plate boundaries. In particular, deep earthquakes (earthquakes that originate below the crust) are found only in association with ocean trenches. Further, earthquakes get deeper with distance from the trench, but only to one side of the trench (the "inland" side). These earthquakes mark the descent of cold, rigid oceanic lithosphere into the asthenosphere. Normally earthquakes are not possible in the asthenosphere because the asthenosphere is a soft solid close to its melting temperature. But where subduction occurs, cold, rigid oceanic lithosphere (the subducting plate) is sinking into the asthenosphere. The subducting plate remains cold enough and rigid enough to produce earthquakes to depths up to 700 km. This is how the process of subduction, and the places it occurs, was discovered - by the relationship of deep earthquakes to trenches.)

Describe the classification of igneous rocks.

Igneous rocks are classified on the basis of composition and texture. Composition refers to the types and relative abundances of minerals present. Felsic igneous rocks are those with lots of light colored minerals and are rich in elements like silica, sodium, and potassium. Mafic igneous rocks are rich in dark minerals containing iron and magnesium, while intermediate igneous rocks have more equal proportions of light and dark minerals. Texture refers to the size and arrangement of mineral crystals in the rock and reflects how the rock formed. There are two basic types of igneous rock textures: intrusive, also called phaneritic, igneous rocks have larger mineral crystals because they cooled slowly underground. Extrusive (aphanitic) igneous rocks are fine grained because they cooled quickly at the surface. Other igneous textures include pyroclastic (explosive), vesicular (bubbly), and vitreous (glassy).

Explain the relationships between magma composition, crustal thickness, fractional crystallization, and tectonic setting.

Basalt, which is low in silica like the mantle, can be produced in any setting where tectonic processes cause the mantle to melt - ocean ridges, continental rifts, or hot spots (melting by decompression) and also subduction zones (melting by addition of water). Andesite, with an intermediate amount of silica, is usually associated with subduction zones and rhyolite, with a high amount of silica, can only be found on continental crust. These higher silica compositions can be produced by melting of silica rich parent materials or by fractional crystallization or crustal contamination of basalt. This occurs where the crust is thicker - such as in subduction zones - where the basalt takes time to work its way through the overriding plate - resulting in fractional crystallization that produces enrichment in silica. Further enrichment in silica can occur where the crust is composed of high silica continental crust, which partially melts and adds high silica to the original mantle-derived basalt magma to produce andesite or rhyolite, such as a volcanic arc built atop continental crust (example: Cascades), or when a continental plate passes over a hot spot (example: Yellowstone). (Magmas originate in the mantle. Because the mantle is mafic (fe/mg rich, silica poor), melting of the mantle produces mafic magma - basalt. In tectonic settings where the crust is thin - oceanic ridges, continental rifts, oceanic hotspots, the magma erupts unchanged as basalt. Where the crust is thick, fractional crystallization produces more felsic magma compositions - andesite and rhyolite -- by the time the magma erupts at the surface. More felsic magma compositions are only seen where the crust is thick - subduction zones, continental hotspots.)

Earth's atmosphere was once very similar in composition to the atmosphere of

Both B and C: Venus and Mars

The division of Earth's interior into core, mantle, and crust is used to describe differences in

Composition. (We use the crust-mantle-core terminology to describe differences in chemical composition in Earth's interior. We use the lithosphere- asthenosphere terminology to describe differences the physical state and behavior. Note that the boundaries between layers with different compositions do not occur at the same depths as the boundaries between layers with different physical properties. For example, there is a change in composition when passing from crust into mantle. But there is no change in physical state or behavior at this boundary- the uppermost part of the mantle behaves in the same way that the crust does because it is still far enough below its melting temperature to be a rigid, brittle solid. The transition to a softer, more pliable solid occurs further down within the mantle and this change in behavior occurs with no change in composition.)

What types of volcanoes are associated with eruptions of basaltic magma? Name one example of each of these types of volcanoes, and explain how their shapes are related to their eruptive style.

Eruptions of basaltic magma produce shield volcanoes, such as Mauna Loa in Hawaii and cinder cones, such as Sunset Crater in Arizona. Basaltic magma is low in silica and so has a low viscosity, is very fluid, and flows easily. Eruptions of thin, fluid basaltic lavas produce broad, gently sloping shield volcanoes like Mauna Loa. If the basaltic magma is charged with gases, it produces a different style of eruption and type of volcano called a cinder cone. Gas rich basaltic magmas have frothy eruptions which hurl material through the air, which cools and piles up around the erupting vent. This forms a conical pile of loose cinders around the vent called a cinder cone.

Describe the physical properties that you would use to distinguish A) halite from calcite and B) galena from graphite.

Halite and calcite are both white to clear, but have different cleavages and crystal shapes. Halite has cubic cleavage, with three cleavage directions at 90 degrees, giving the mineral the shape of a cube. Calcite also has three cleavage directions, but they are not at 90 degrees so the mineral has a rhombic shape. Halite also has a distinct salty taste, while calcite while fizz if you put acid on it. Galena and graphite are both gray to black in color, but galena has a very bright metallic luster while graphite's luster is more dull, giving the minerals very different appearances. Galena also has three cleavage directions at right angles, while graphite has only one. While both graphite and galena are low in hardness, graphite is softer and is in fact the mineral with the lowest hardness on Moh's scale. (Please explain how the cleavage and habit is different for each pair of minerals. Also, galena is much denser than graphite and has a brighter metallic luster.)

Elements are arranged in the periodic table by

their number of protons the filling of electrons in different orbitals and energy levels their atomic number

How did the discovery of magnetic reversals confirm the sea floor spreading hypothesis?

Magnetic reversals are the periodic change in the orientation of earth's magnetic field. For reasons not fully understood, the magnetic field occasionally flips polarity, in other words, magnetic north becomes magnetic south and vice versa. When rocks containing iron minerals crystallize from lava, the iron mineral crystals align themselves with the polarity of the ambient magnetic field at the time of crystallization. This orientation is locked in place and will not change. When the seafloor was mapped in detail, it was discovered that the oceanic exhibits "stripes" of normally and reversely magnetized rocks that are symmetric on either side of the mid-ocean ridge system. In other words, rocks at the ridge axis today show normally polarity, and then as you move away from the ridge on either side you enter a belt of reversely magnetized rocks, and so on. This pattern indicates that rocks along the ridge today formed in today's interval of normal polarity. Rocks in reversely magnetized belt formed in the last period of reserved polarity, and then moved away from the ridge due to seafloor spreading.

Melting of rocks to produce magma can be caused by

Raising the temperature Adding water Lowering the pressure

Eruption of rhyolite magma tends to produce

lava domes (rhyolite magma is high in viscosity (meaning it is thick and pasty) by virtue of its high silica content and low temperature (compared to basalt). The high viscosity of rhyolite prevents it from flowing away from the vent where it is erupted. Instead, rhyolite piles up are the vent forming a steep, dome shaped structure - a lava dome. Meanwhile, shield volcanoes and cinder cones are associated with eruptions of low viscosity basalt, either effusively when little gas is present (shield) or a foamy or frothy manner when large amounts of gas are present (cinder cones). Intermediate viscosity andesite produces composite volcanoes that erupt both explosively (when the magma is gas charged) and effusively (after the magma has de-gassed).)

Oceanic crust is produced by seafloor spreading. How is continental crust produced?

both of the above (Continental crust is produced at subduction zones as buoyant materials on the subducting plate are accreted to the leading edge of the overriding plate, while the remainder of the plate (due to its higher density) is subducted into the mantle. Introduction of water into the mantle by the subducting plate causes melting in the upper mantle producing magmas that are richer in silica and more like the continental crust than they are the mantle or the subducting plate. These magmas may erupt at the surface but most crystallize within the crust of the overriding plate as intrusion. Either way, more material is added to the continental plate. Both terrane accretion and partial melting makes continental crust by separating out buoyant materials from the subducting plate.)

Rhyolite magma

can be formed by fractional crystallization of basalt magma (Silica content is positively correlated with magma viscosity. This means that high silica rhyolite (65-75% silica) is also high in viscosity. This causes it to erupt explosively if gases are present in the magma because the high viscosity nature of the magma (thick, pasty) makes it difficult for gases to be released from the magma. High silica magma compositions can be produced from basalt (the magma composition produced by melting of the upper mantle) through fractional crystallization. As the magma cools, mafic minerals high in Fe, Mg, and Ca are crystallized first by virtue of their higher melting points. This results in a smaller volume of more felsic magma - magma enriched in silica, Na, and K - such as andesite and rhyolite. The amount of silica enrichment depends on the amount of fractional crystallization that has occurred.)

Which list of igneous rocks is in order of increasing silica content?

gabbro, andesite, rhyolite (Mafic igneous rocks such as basalt (effusive) and gabrro (intrusive) have the lowest silica (50%) and most iron/magnesium. Intermediate rocks like andesite (extrusive) and diorite (intrusive) have more silica (60%) and a more even mix of Fe/Mg/Ca and Na/K. Felsic rocks like rhyolite (effusive) and granite (intrusive) have the most silica, the least Fe, Mg, and Ca, and the most Na and K.)

A caldera

results from the collapse of a volcano after its magma chamber empties

How did seafloor spreading revive Alfred Wegener's ideas about continental drift?

sea floor spreading provided a viable mechanism for moving the continents

At a passive continental margin the primary geologic process occuring is

sedimentation (Passive continental margins are places where the crust changes from thick continental crust to thin oceanic crust, but functioning as one plate- moving at the same direction and speed - and as such do not coincide with a plate boundary. When that area of continental crust first started breaking up by continental rifting, it was at a new divergent boundary, but is now far from the plate boundary due to addition of new oceanic crust to the edge of each plate by seafloor spreading- with the plate boundary remaining at the ocean ridge. As the edge of the continental crust gets further and further from the divergent plate boundary where hot mantle rock is rising, the continental crust cools and subsides (having been heated and elevated during the rifting stage). Since the crust was also thinned by the rifting process, its not as thick as it once was and drops below sea level as it cools, forming a broad continental shelf. This provides a place for large amounts of sediment to accumulate - this sedimentation is the primary geologic process occurring in a passive margin setting.)

Describe the Earth's internal structure in terms of both chemical composition and physical properties.

The earth is divided into three layers, a crust, a mantle and a core, that differ in their chemical composition. The crust is the thin outer layer, consisting primarily of oxygen, silicon and aluminum. It is further subdivided into continental and oceanic crust. The oceanic crust is thinner and denser, and is similar in composition to basalt (Si, O, Ca, Mg, and Fe). The continental crust is thicker and less dense, and is similar to granite in composition (Si, O, Al, K, and Na). The mantle is made of magnesium, iron and silicon. The core is almost exclusively iron and nickel. The outer core is liquid iron and the inner core is solid iron. The mantle and crust are further divided into the lithosphere, asthenosphere and mesosphere, depending on their physical properties, namely how close the material is to its melting point. The lithosphere is cold and rigid and includes the curst and uppermost part of the mantle where rocks are far below their melting point. Further down in the mantle is the asthenosphere where rocks are weak and can flow because they are close to their melting point. The deepest part of the mantle is the mesosphere where rocks are gain strong and below their melting point due to the high pressure.

How does the lithosphere differ from the asthenosphere?

The lithosphere is a rigid and brittle solid while the asthenosphere is a soft and ductile solid. (The terms crust and mantle are used to denote differences in chemical composition; while the terms lithosphere and asthenosphere are used to denote differences in physical state (for example, rigid, brittle solid vs.. soft, ductile solid). Because the boundaries between layers with different chemical compositions do not occur at the same depth as boundaries between layers with different physical properties, lithosphere cannot be considered synonymous with crust, not can asthenosphere be considered synonymous with mantle. For example, a large change in chemical composition occurs as you move from crust to mantle. However, there is no change in physical properties at boundary between crust and mantle because uppermost mantle is still cold enough relative to its melting point to behave in the same manner as the crust- as a rigid brittle solid. The transition to a soft ductile solid- the asthenosphere- does not occur until we get further down into the upper mantle - where temperature approaches the melting point of the mantle at that depth.)

Identify the three basic types of plate boundaries and describe the major geologic and tectonic processes occurring at each.

The three types of plate boundaries are convergent, divergent, and transform. At convergent plate boundaries, two plates are colliding with each other. This results in subduction and destruction of old, dense oceanic crust, volcanism, mountain building, and earthquakes. At divergent plate boundaries, two plates are moving away from each other. This allows magma from the mantle to rise towards the surface and form new oceanic crust. This process is known as seafloor spreading. At transform plate boundaries, two plates are sliding past each other in different directions. No crust is being created or destroyed at transform plate boundaries, but earthquakes do occur.